1. Field of the Invention
The present invention relates to a laser beam to wave length conversion through use of a nonlinear optical crystal, and more particularly, to a technique for processing a workpiece by emitting a wavelength-converted laser beam on the workpiece.
2. Description of the Related Art
In a wavelength conversion laser of JP-A-5-142607 (pp. 3 to 4, and FIG. 1), when two nonlinear optical crystals are arranged in series with each other, the nonlinear optical crystals are the to be identical in length in a direction in which a laser beam passes through the crystals. One of the nonlinear optical crystals is aligned with an incident laser beam in terms of a crystal orientation axis for phase matching. After having passed through a first nonlinear optical crystal, the incident laser beam enters a second nonlinear optical crystal. Here, the second nonlinear optical crystal is arranged at a position where the crystal orientation axis of the second crystal for phase matching is offset from that of the first crystal by an angle of less than a full width half maximum of a phase matching angle. The offset angle of the crystal axis (crystal orientation axis) of the nonlinear optical crystal is as small as several microradians to tens of microradians (see, e.g., JP-A-5-142607). Specifically, two nonlinear optical crystals, which are identical in length with reference to a direction in which the laser beam passes through the crystals, are arranged such that the crystal orientation axes of the nonlinear optical crystals are directed in substantially the same direction when viewed from the direction of the optical axis of the laser beam.
In a wavelength conversion laser of JP-A-6-110098. (pp. 3 to 4, and FIG. 2), KN crystals, which are nonlinear optical crystals, each have a thickness of 0.5 mm. When the two KN crystals are arranged while cut angles θ of the crystals are changed from each other, orientations of crystallographic axes (crystal orientation axes) of the crystals are arranged so as to compensate for a walking angle (see, e.g., JP-A-6-110098). Specifically, two nonlinear optical crystals, which are identical in length with reference to a direction in which a laser beam passes through the crystals, are arranged such that crystal orientation axes of the respective nonlinear optical crystals are rotated substantially through 180° around the optical axis of the laser beam when viewed from the direction of the optical axis.
A wavelength conversion laser of JP-A-4-330425 (pg. 3, and FIG. 1) uses two identical crystals of nonlinear optical crystalline material. When the crystals are made equal in length to each other, the crystals are arranged such that the mutually-equivalent optical axes of the crystals are offset from each other by 90° (see, e.g., JP-A-4-330425).
In the wavelength conversion laser beams of the JP-A-5-142607 and JP-A-6-110098, the two nonlinear optical crystals that are identical in length with reference to the direction in which the laser beam passes through the crystals are arranged such that the crystal orientation axes of the respective nonlinear optical crystals are orientated in substantially the same direction when viewed from the direction of optical axis of the laser beam. Alternatively, the two nonlinear optical crystals are arranged such that the crystal orientation axes of the respective nonlinear optical crystals are rotated about 180° around the optical axis. Consequently, the polarizing directions of the wavelength-converted laser beams emitted from the two nonlinear optical crystals are aligned in the same direction; namely, the laser beams are output as linearly-polarized wavelength-converted laser beams. Therefore, the wavelength-converted laser-beams emitted from the two nonlinear optical crystals cause interference. Now, the refractive index of a gas, such as air, present between the two nonlinear optical crystals or the refractive indices of antireflection coatings (AR coatings) provided on the end faces of the two nonlinear optical crystals are susceptible to wavelength dispersion. Conditions for occurrence of interference between the wavelength-converted laser beams emitted from the nonlinear optical crystals change according to a distance between the two nonlinear optical crystals. If two nonlinear optical crystals fail to be spaced at a specific interval, generation of a high-output wavelength-converted laser beam is impossible. Since the refractive index of the air existing between the two nonlinear optical crystals or the refractive index of the antireflection coating changes according to a temperature, conditions for interference vary according to temperatures, and hence there arises a problem of resultantly-generated wavelength-converted laser beams becoming very unstable.
The wavelength conversion laser according to the JP-A-4-330425 has a configuration for effecting wavelength conversion within a resonator for generating a fundamental laser beam. The laser beam enters a nonlinear optical crystal while traveling back and forth within the resonator. In order to cancel an offset between the fundamental laser beams emitted every time the laser beam passes through the nonlinear optical crystals, there has been a necessity for making the two nonlinear optical crystals strictly identical in length with respect to the direction in which the laser beam passes. Therefore, when the nonlinear optical crystals are exchanged, there arises a necessity for preparing and exchanging a nonlinear optical crystal strictly identical in length with the original nonlinear optical crystal or preparing and exchanging two nonlinear optical crystals strictly identical in length. This raises a problem of a hike in maintenance costs and a problem of an increase in the time to exchange and adjust the nonlinear optical crystals.
Under an existing laser processing method; for instance, a method for improving the quality of a film by exposing amorphous silicon to a laser beam, a double waveform of a Q-switch YAG laser is radiated on amorphous silicon (see, e.g., JP-A-63-314862 (pg. 2, FIGS. 1, 3).
Under another existing laser processing method; for instance, a method for crystallizing amorphous silicon by exposing amorphous silicon to a laser beam, a laser beam is radiated while a fundamental wave, a second harmonic, a third harmonic, or a fourth harmonic of a YAG laser is modified such that the profile of an exposed surface becomes linear during the course of exposure (see, e.g., JP-A-2001-144027 (pp. 4 to 5, FIG. 2)).
According to the related-art laser processing method, when a wavelength-converted laser beam originating from a wavelength conversion laser equipped with a wavelength converter is used for exposure, a wavelength-converted laser emitted from the wavelength conversion laser (i.e., the wavelength converter) is usually linearly-polarized, which raises a problem of occurrence of a difference in processing results depending on a processing direction and a polarizing direction. Particularly, in the case of processing operation for realizing polysilicon by radiating a laser beam on amorphous silicon and annealing the thus-exposed silicon, when a device, such as a thin-film transistor, is fabricated through use of a substrate that has been transformed into polysilicon by a conventional laser processing method, there is a problem of a difference arising in characteristics depending on a relationship between the scanning direction and polarizing direction of the laser beam.